Electrolyte polymer for polymer electrolyte fuel cells, process for its production and membrane-electrode assembly

ABSTRACT

To provide an electrolyte polymer for polymer electrolyte fuel cells, made of a perfluorinated polymer having sulfonic groups, characterized in that in a test of immersing 0.1 g of the polymer in 50 g of a fenton reagent solution containing 3% of an aqueous hydrogen peroxide solution and 200 ppm of bivalent iron ions at 40° C. for 16 hours, the amount of eluted fluorine ions detected in the solution is not more than 0.002% of the total amount of fluorine in the polymer immersed. The electrolyte polymer of the present invention has very few unstable terminal groups and has an excellent durability, and therefore, is suitable as a polymer constituting an electrolyte membrane for polymer electrolyte fuel cells and a polymer contained in a catalyst layer.

This application is a Divisional of U.S. application Ser. No.11/271,915, filed on Nov. 14, 2005, which is a Continuation ofPCT/JP04/06689, filed on May 12, 2004.

TECHNICAL FIELD

The present invention relates to an electrolyte polymer for polymerelectrolyte fuel cells and a membrane-electrode assembly for polymerelectrolyte fuel cells.

BACKGROUND ART

Attention has been drawn to a hydrogen-oxygen fuel cell as a powergenerating system which presents substantially no adverse effects on theglobal environment because in principle, its reaction product is wateronly. Polymer electrolyte fuel cells were once mounted on spaceships inthe Gemini project and the Biosatellite project, but their powerdensities at the time were low. Later, more efficient alkaline fuelcells were developed and have dominated the fuel cell applications inspace including space shuttles in current use.

Meanwhile, with the recent technological progress, attention has beendrawn to polymer fuel cells again for the following two reasons: (1)Highly ion-conductive membranes have been developed as polymerelectrolytes and (2) it has been made possible to impart extremely highactivity to the catalysts for use in gas diffusion electrodes by usingcarbon as the support and incorporating an ion exchange resin in the gasdiffusion electrodes so as to be coated with the ion exchange resin.

However, a perfluorinated polymer having sulfonic groups to be used as apolymer contained in a membrane and an electrode usually has unstablefunctional groups such as —COOH groups, —CF═CF₂ groups, —COF groups and—CF₂H groups at some molecular chain terminals, and therefore, there wassuch a problem that a polymer gradually decomposes during long-term fuelcell operations, followed by decreasing the power generation voltage. Inaddition, there was such a problem that the fuel cell operation cannotbe conducted because decrease of the mechanical strength due to thepolymer decomposition, locally causes pinholes, breaking, abrasion orthe like.

The above problems are caused by the presence of such unstablefunctional groups at some molecular chain terminals of afluorine-containing polymer, and as methods for stabilizing suchmolecular chain terminals, for example, the following methods have beenproposed.

A method of hydrothermal treatment of atetrafluoroethylene/hexafluoropropylene copolymer (hereinafter referredto as a TFE/HFP copolymer) at a high temperature to convert —COOH groupsto —CF₂H groups (U.S. Pat. No. 3,085,083).

A method of decarboxination and fluorination of a fluorine-containingpolyether having a low molecular weight by using fluorine gas in aliquid state or a state as dissolved in an inert solvent, to stabilizeterminal groups (U.S. Pat. No. 3,242,218).

A method of shearing a TFE/HFP copolymer by a twin-screw extruder at ahigh temperature, followed by treating with fluorine gas (U.S. Pat. No.4,626,587).

A method of treating a tetrafluoroethylene/perfluoroalkyl vinyl ethercopolymer (hereinafter referred to as a TFE/PFVE copolymer) bycontacting it with fluorine gas in the form of pellets (JP-B-4-83).

A method of treating a TFE/PFVE copolymer by contacting it with fluorinegas in the form of granules (JP-B-7-30134).

A method of treating a TFE/HFP copolymer or a TFE/PFVE copolymer bycontacting it with fluorine gas in the form of a pulverized producthaving an average particle diameter of from 5 to 500 μm (JP-B-7-5743).

A method of treating a TFE/PFVE copolymer by stirring a polymerizationproduct obtained by solution polymerization or suspension polymerizationin water, followed by contacting the resulting spherical granules havingan average particle diameter of from 1 to 5 mm with fluorine gas(JP-A-10-87746).

A method of subjecting a TFE/HFP copolymer or a TFE/PFVE copolymer toreactive heat treatment with oxygen and water by a kneader(JP-A-2000-198813).

A method of carrying out treatment of a TFE/HFP copolymer or a TFE/PFVEcopolymer by melt-kneading in the presence of oxygen and melt-kneadingin the presence of water in a single kneader (JP-A-2002-249585).

However, such methods are not designed for treatment of a polymer havingion exchange groups or their precursor groups, but designed forstability of a fluorine-containing polymer at the time of heat forming.Here, in this specification, precursor groups for ion exchange groupsmean groups convertible to ion exchange groups by e.g. hydrolysis, andprecursor groups for sulfonic groups may, for example, be —SO₂F groupsor —SO₂Cl groups.

As a method of improving the stability of a fluorine-containing polymercontaining ion exchange groups or their precursor groups, a treatingmethod has been proposed wherein a perfluoropolymer having sulfonicgroups is put in a shaking tube coated with nickel or a stainless steelcontainer and contacted with fluorine gas (JP-B-46-23245). However, bysuch a method, the treatment was not sufficient, and if aperfluoropolymer having sulfonic groups, treated by such a method, wasused, there was a problem that although the voltage decrease in a fuelcell operation became small, it did not reach a level of at most 10μV/h, and the sufficient durability could not be obtained.

Further, in such a treating method with fluorine gas, a peroxide test isdescribed as an index for durability against polymer decomposition,wherein from 0.5 to 1.5 g of a polymer is immersed in 50 g of a fentonreagent solution containing 30% of an aqueous hydrogen peroxide solutionand 10 ppm of bivalent iron ions at 85° C. for 20 hours, and the weightdecrease is measured after drying. However, there was a problem that apolymer containing ion exchange groups is highly hygroscopic and cannotbe measured with sufficient precision.

DISCLOSURE OF THE INVENTION

Accordingly, it is an object of the present invention to provide anelectrolyte polymer for polymer electrolyte fuel cells excellent indurability, which is obtained by reducing the number of unstablefunctional groups present at some molecular chain terminals of afluorine-containing polymer having sulfonic groups to be used as anelectrolyte polymer contained in electrolyte membranes and catalystlayers for polymer electrolyte fuel cells.

The present invention provides an electrolyte polymer for polymerelectrolyte fuel cells, made of a perfluorinated polymer having sulfonicgroups, characterized in that in a test of immersing 0.1 g of thepolymer in 50 g of a fenton reagent solution containing 3% of an aqueoushydrogen peroxide solution and 200 ppm of bivalent iron ions at 40° C.for 16 hours, the amount of eluted fluorine ions detected in thesolution is not more than 0.002% of the total amount of fluorine in thepolymer immersed.

Further, the present invention provides a process for producing anelectrolyte polymer for polymer electrolyte fuel cells made of aperfluorinated polymer having sulfonic groups, characterized in that aperfluorinated polymer having precursor groups for sulfonic groups, issubjected to heat treatment for at least 0.1 hour at a temperature offrom 200 to 300° C. under a reduced pressure of at most 0.02 MPa andthen contacted with fluorine gas at a temperature of from 150 to 200° C.and further subjected to hydrolysis and treatment for conversion to anacid form, to convert the precursor groups to sulfonic groups.

Further, the present invention provides a membrane-electrode assemblyfor polymer electrolyte fuel cells, which comprises an anode and acathode each having a catalyst layer comprising a catalyst and anelectrolyte polymer, and an electrolyte membrane disposed therebetween,characterized in that at least one polymer among the polymerconstituting the electrolyte membrane, the polymer contained in theanode catalyst layer and the polymer contained in the cathode catalystlayer, is made of the above electrolyte polymer.

Further, the present invention provides a method for producing amembrane-electrode assembly for polymer electrolyte fuel cells, whichcomprises an anode and a cathode each having a catalyst layer comprisinga catalyst and an electrolyte polymer, and an electrolyte membranedisposed therebetween, characterized in that at least one polymer amongthe polymer constituting the electrolyte membrane, the polymer containedin the anode catalyst layer and the polymer contained in the cathodecatalyst layer, is produced by the above process.

The electrolyte polymer of the present invention is a perfluorinatedpolymer having very few unstable terminal groups, and therefore, in acase where such a polymer is used as a membrane for fuel cells orcontained in catalyst layers, the polymer decomposition due to a fuelcell operation can be suppressed. As a result, a polymer electrolytefuel cell excellent in durability can be provided.

Further, according to the production process of the present invention,the perfluorinated polymer having sulfonic groups can be subjected tofluorination treatment efficiently and sufficiently, whereby anelectrolyte polymer having very few unstable terminal groups asmentioned above can efficiently be obtained.

BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, as an index of durability againstdecomposition of the polymer in the fuel cell operation, an immersiontest using a fenton reagent is employed. This test is carried out byimmersing the polymer in an aqueous solution containing an aqueoushydrogen peroxide solution and bivalent iron ions. The concentration ofthe aqueous hydrogen peroxide solution is from 1 to 30%, the ionconcentration of the bivalent iron ions is from 10 to 500 ppm, theimmersion temperature is from 25 to 90° C., and the immersion time isfrom 0.5 to 24 hours. The polymer undergoes a slight weight reduction bythe polymer decomposition caused by hydroxy radicals or hydroperoxyradicals which are produced in the fenton reagent. However, in the caseof a polymer having ion exchanging groups, the polymer is highlyhygroscopic, which makes it difficult to measure the weight preciselyeven when the polymer is dried. Accordingly, it is preferred from theviewpoint of sensitivity to detect fluorine ions eluted in the fentonreagent solution at the time of decomposition.

In the present invention, the test is carried out, in which 0.1 g of theelectrolyte polymer is immersed in 50 g of a fenton reagent solutioncontaining 3% of an aqueous hydrogen peroxide solution and 200 ppm ofbivalent iron ions at 40° C. for 16 hours. The electrolyte polymer ofthe present invention is such that in the above test, the amount ofeluted fluorine ions detected in the solution is not more than 0.002% ofthe total amount of fluorine in the polymer immersed. If it is more than0.002%, the amount of the unstable terminal groups is large, whereby thevoltage is likely to decrease during a long-term fuel cell operation.

The electrolyte polymer for polymer electrolyte fuel cells in thepresent invention may, for example, be a copolymer of a perfluorovinylcompound represented by the formulaCF₂═CF(OCF₂CFX)_(m)—O_(p)—(CF₂)_(n)SO₃H (wherein X is a fluorine atom ora trifluoromethyl group, m is an integer of from 0 to 3, n is an integerof from 0 to 12, and p is 0 or 1, provided that when n=0, p=0) with aperfluoroolefin, a perfluoroalkyl vinyl ether or the like. Specificexamples of the above perfluorovinyl compound are compounds representedby the formulae 1 to 4. Here, in the formulae 1 to 4, q is an integer offrom 1 to 9, r is an integer of from 1 to 8, s is an integer of from 0to 8, and z is 2 or 3.CF₂═CFO(CF₂)_(q)SO₃H  Formula 1CF₂═CFOCF₂CF(CF₃)O(CF₂)_(r)SO₃H  Formula 2CF₂═CF(CF₂)_(s)SO₃H  Formula 3CF₂═CF[OCF₂CF(CF₃)]_(z)O(CF₂)₂SO₃H  Formula 4

The polymer comprising repeating units based on a perfluorovinylcompound having a sulfonic group is usually obtained by polymerizationof a perfluorovinyl compound having a —SO₂F group. The perfluorovinylcompound having the —SO₂F group may be homopolymerized, but it isusually copolymerized with a comonomer such as a perfluoroolefin or aperfluoro(alkyl vinyl ether) as mentioned above because it is unlikelyto undergo radical polymerization. The perfluoroolefin to be used as acomonomer, may, for example, be tetrafluoroethylene orhexafluoropropylene. Usually, it is preferred to usetetrafluoroethylene.

The perfluoro(alkyl vinyl ether) to be used as a comonomer is preferablya compound represented by CF₂═CF—(OCF₂CFY)_(t)—O—R^(f). Here, in theformula, Y is a fluorine atom or a trifluoromethyl group, and t is aninteger of from 0 to 3. R^(f) is a linear or branched perfluoroalkylgroup represented by C_(u)F_(2u+1) (1<u<12).

Preferred examples of the compound represented byCF₂═CF—(OCF₂CFY)_(t)—O—R^(f) may be compounds represented by theformulae 5 to 7. Here, in the formulae 5 to 7, v is an integer of from 1to 8, w is an integer of from 1 to 8, and x is an integer of from 1 to3.CF₂═CFO(CF₂)_(v)CF₃  Formula 5CF₂═CFOCF₂CF(CF₃)O(CF₂)_(w)CF₃  Formula 6CF₂═CF[OCF₂CF(CF₃)]_(x)O(CF₂)₂CF₃  Formula 7

In addition to a perfluoroolefin or a perfluoro(alkyl vinyl ether),another perfluorinated monomer such as perfluoro(3-oxahepta-1,6-diene)may also be copolymerized with the perfluorovinyl compound having a—SO₂F group, as a comonomer.

In a case where the electrolyte polymer of the present invention is tobe used as an electrolyte membrane for polymer electrolyte fuel cells,the concentration of the sulfonic groups in the electrolyte polymer,namely, the ion exchange capacity, is preferably from 0.5 to 2.0 meq/9dry resin, particularly preferably from 0.7 to 1.6 meq/9 dry resin. Ifthe ion exchange capacity is lower than such a range, the resistance ofthe electrolyte membrane obtainable tends to be large. On the otherhand, if it is higher than such a range, the mechanical strength of theelectrolyte membrane tends to be insufficient.

The process for producing the electrolyte polymer of the presentinvention is characterized in that a perfluorocarbon polymer (which maycontain an oxygen atom of an ether bond type) having precursor groupsfor sulfonic groups, is subjected to heat treatment for at least 0.1hour at a temperature of from 200 to 300° C. under a reduced pressure ofat most 0.02 MPa and then contacted with fluorine gas at a temperatureof from 150 to 200° C. Here, the precursor groups for the sulfonicgroups are mainly —SO₂F groups. Unstable terminal groups such as —COOHgroups or —CF═CF₂ groups present in some molecular chain terminals arefirstly converted to —COF groups by heat treatment, and then convertedto stable —CF₃ groups by contacting with fluorine gas. It is consideredthat the heat treatment under reduced pressure in a first step has aneffect of accelerating the conversion of functional groups although itis not obvious, and the treatment of contacting it with fluorine gas ina second step can increase the ratio of conversion to the stable —CF₃groups.

The heat treatment temperature under reduced pressure in the presentinvention is usually from 200 to 300° C., preferably from 220 to 280° C.If it is lower than 200° C., conversion of the unstable functionalgroups tends to be insufficient, such being undesirable. On the otherhand, if it is higher than 300° C., precursor groups (—SO₂F groups) forthe ion exchange groups tend to be decomposed during such treatment,thus leading to a decrease of the ion exchange capacity of the finallyobtainable electrolyte polymer. It is particularly preferably from 220to 280° C., whereby conversion of the unstable functional groups willtake place efficiently, while no decomposition of the —SO₂F group willtake place.

The pressure in the heat treatment under reduced pressure is preferablyat most 0.02 MPa, more preferably at most 0.01 MPa. If it is more than0.02 MPa, conversion of the unstable terminal functional groups will nottake place efficiently, such being undesirable. The heat treatment ispreferably carried out under a pressure of at most 0.01 MPa, wherebyconversion efficiency of the unstable terminal functional groups becomesremarkably high. The treatment time is usually at least 0.1 hour,preferably from 0.2 to 16 hours. If it is less than 0.1 hour, conversionof the unstable functional groups will not take place sufficiently, suchbeing undesirable. If it is more than 16 hours, such will bedisadvantageous from the viewpoint of productivity. It is preferablyfrom 0.2 to 16 hours, whereby the conversion of the unstable functionalgroups will be sufficient and the productivity can also be secured.

The above heat treatment under reduced pressure may be carried out in areduced pressure oven, but may efficiently be carried out by means of akneader such as a twin-screw extruder. In the case of using a reducedpressure oven, such heat treatment is preferably carried out by thinlyand uniformly dispersing a polymer powder to be treated, on afluorine-containing heat-resistant sheet of e.g. a perfluoroalkoxy ether(PFA). By such heat treatment, the polymer powder will be melted andformed into a sheet. The thickness of the sheet after the heat treatmentis preferably at most 5 mm, whereby subsequent fluorine gas treatmentwill be sufficiently carried out. The thickness is further preferably atmost 2 mm, whereby sufficient fluorination treatment can be carried outin a short period of time.

In the present invention, the fluorination treatment for contact withfluorine gas is usually carried out at a temperature of from 150 to 200°C., preferably from 170 to 190° C. If it is lower than 150° C.,conversion of the —COF groups to the —CF₃ groups will not besufficiently carried out. If it is higher than 200° C., decomposition ofthe precursor groups (—SO₂F groups) is likely to take place and the ionexchange capacity of the finally obtainable electrolyte polymer tends tobe small, such being undesirable. Fluorine gas is contacted preferablyat a temperature of from 170 to 190° C., whereby no decomposition of the—SO₂F groups will take place and the conversion into the —CF₃ groupswill take place efficiently and sufficiently. The reaction of fluorinegas is a drastic exothermic reaction, and from the viewpoint of safety,the fluorine gas to be used is preferably diluted with inert gas such asnitrogen, and the pressure is preferably at a level of at most 1 MPa.After the fluorination treatment, the temperature is lowered, andunreacted fluorine gas is removed.

A reactor to be used at the time of contacting the polymer with fluorinegas is preferably a pressure-resistant reactor having an inside surfacemade of hastelloy C alloy. The reason is not clearly understood, butwhen a pressure-resistant reactor having an inside surface made ofhastelloy C alloy is used, the conversion efficiency of the terminalunstable functional groups into stable functional groups at the time offluorination treatment becomes high, such being desirable.

For example, if the copolymer comprising repeating units based ontetrafluoroethylene and repeating units based on a perfluorovinylcompound having a sulfonic group, is produced without being subjected tothe above heat treatment under reduced pressure and fluorinationtreatment in its production process, the amount of eluted fluorine ionsdetected in the solution in a test with a fenton reagent, is usually atleast 0.05% of the total amount of fluorine in the polymer immersed.However, such an amount can be suppressed to a level of not more than0.002% by carrying out the above heat treatment under reduced pressureand fluorination treatment.

The electrolyte polymer of the present invention can suitably be used asa polymer constituting an electrolyte membrane or an electrolyte polymercontained in an anode and a cathode for polymer electrolyte fuel cells.It is particularly preferred that the electrolyte polymer of the presentinvention is used as both the polymer constituting the electrolytemembrane and the electrolyte polymer contained in the anode and thecathode, from the viewpoint of durability of fuel cells. The polymerelectrolyte fuel cell has a membrane-electrode assembly having a cathodeand an anode disposed respectively on both sides of the electrolytemembrane, and a fuel is supplied to this membrane-electrode assembly forpower generation. Such cathode and anode are usually made of a catalystlayer and a gas diffusion layer. The catalyst layer is a layercontaining a catalyst and an electrolyte polymer and is disposedadjacent to the electrolyte membrane. The gas diffusion layer is aporous layer disposed adjacent to the catalyst layer, and has a role ofefficiently supplying gas to the catalyst layer and a role as a currentcollector. Usually, a carbon cloth or the like is used for the gasdiffusion layer.

Now, the present invention will be described in further detail withreference to Examples and Comparative Examples. However, it should beunderstood that the present invention is by no means restricted thereto.

EXAMPLE 1

2,800 g of a copolymer powder comprising repeating units based ontetrafluoroethylene and repeating units based onCF₂═CF—OCF₂CF(CF₃)O(CF₂)₂SO₂F (ion exchange capacity measured asconverted into an acid form: 1.1 meq/g dry resin, hereinafter referredto as copolymer A) was uniformly dispersed on a PFA sheet, followed byheat treatment in a reduced pressure oven under a pressure of 10 Pa at atemperature of 250° C. for 4 hours. The thickness of a molten sheetafter the heat treatment was 2 mm. The infrared absorption spectrabefore and after the heat treatment under reduced pressure werecompared, whereby the absorption attributable to a —COOH group at 1,780cm⁻¹ and 1,810 cm⁻¹ and the absorption attributable to —CF═CF₂ at 1,800cm⁻¹ were found to be decreased, and the absorption attributable to a—COF group at 1,880 cm⁻¹ was found to be increased by the heat treatmentunder reduced pressure.

On the other hand, into a pressure-resistant reactor having an innercapacity of 32 L and having an inside surface made of a hastelloy Calloy, a multistage shelf made of a hastelloy C alloy was put, and amixed gas consisting of 20% of fluorine gas and 80% of nitrogen gas wasintroduced under a gage pressure of 0.25 MPa. The reaction system wasmaintained at 190° C. for 4 hours to carry out passivation treatment ofthe metal surface. After lowering the temperature, the sheet which wassubjected to the above heat treatment under reduced pressure, was put onthe shelf in the above 32 L pressure-resistant reactor, and a mixed gasconsisting of 20% of fluorine gas and 80% of nitrogen gas was introducedunder a gage pressure of 0.25 MPa. The reaction system was maintained at180° C. for 4 hours to carry out fluorination treatment. After thetreatment, fluorine gas was discharged and a polymer was taken out andpulverized by a pulverizer to obtain a fluorination-treated polymerhaving —SO₂F groups as precursor groups for sulfonic groups (hereinafterreferred to as a precursor polymer).

The above fluorination-treated precursor polymer was hydrolyzed in anaqueous solution containing 20% of methanol and 10% of potassiumhydroxide, and then, it was washed with sulfuric acid to be converted toan acid form and further washed with deionized water, to convert —SO₂Fgroups to sulfonic groups thereby to obtain an acid form polymer.

The polymer obtained was maintained for 24 hours in a glove box suppliedwith nitrogen. Then, about 0.1 g of the polymer was weighed in the glovebox and immersed in 50 g of a fenton reagent solution containing 3% ofan aqueous hydrogen peroxide solution and 200 ppm of bivalent iron ionsat 40° C. for 16 hours. After the polymer was removed, the mass of thesolution was measured, and the fluorine ion concentration in thesolution was measured by an ion meter, whereupon the amount of elutedfluorine ions was calculated and found to be 0.001% of the total amountof fluorine in the polymer immersed.

EXAMPLE 2

2,500 g of the precursor polymer obtained in Example 1 was kneaded andpelletized by a twin-screw extruder and then molded by extrusion into asheet-form by means of a single-screw extruder to obtain a membranehaving a thickness of 30 μm. The obtained membrane was hydrolyzed byimmersing it in a hydrolytic solution having the same liquid compositionas in Example 1, and after treatment for conversion to an acid form withsulfuric acid, was washed with water to obtain an electrolyte membranefor fuel cells.

On the other hand, 2,500 g of the acid-form polymer obtained in Example1 was dissolved in ethanol by means of a pressure resistant autoclavehaving an inside surface made of hastelloy C alloy, to obtain a 10%ethanol solution of a fluorination-treated copolymer A.

Then, 126 g of distilled water was added to 20 g of a catalyst having50% of platinum supported on a carbon black powder, and ultrasonic waveswere applied for 10 minutes to disperse the catalyst uniformly. 80 g ofthe above 10% ethanol solution of the fluorination-treated copolymer A,was added thereto, and 54 g of ethanol was further added to bring thesolid content concentration to 10%, thereby to obtain a coating liquidfor preparing a cathode catalyst layer. Such a coating liquid wasapplied on a substrate film and dried to form a cathode catalyst layerhaving a platinum amount of 0.5 mg/cm².

Further, 124 g of distilled water was added to 20 g of a catalyst having53% of a platinum/ruthenium alloy (platinum/ruthenium ratio=30/23)supported on a carbon black powder, and ultrasonic waves were appliedfor 10 minutes to disperse the catalyst uniformly. 75 g of the 10%ethanol solution of the above fluorination-treated electrolyte polymer,was added thereto, and 56 g of ethanol was further added to bring thesolid content concentration to about 10%, thereby to obtain a coatingliquid for preparing an anode catalyst layer. Such a coating liquid wasapplied on a substrate film and dried to form an anode catalyst layerhaving a platinum amount of 0.35 mg/cm².

The above fluorination-treated electrolyte membrane for fuel cells, wassandwiched between the cathode catalyst layer and the anode catalystlayer, and pressed by hot press to bond both catalyst layers to themembrane. Then, the substrate films were peeled off to obtain amembrane-catalyst layer assembly having an electrode area of 25 cm². Thepressing conditions were adjusted at a temperature of 120° C., for 2minutes and under a pressure of 3 MPa. Such an assembly was interposedbetween two sheets of gas diffusion layers made of carbon cloth having athickness of 350 μm to prepare a membrane-electrode assembly. Thismembrane-electrode assembly is mounted in a cell for power generation,and a mixed gas (utilization: 70%) consisting of 80% of hydrogen and 20%of carbon dioxide is supplied to the anode and air (utilization: 40%)was supplied to the cathode, respectively at normal pressure. Durabilityof the polymer electrolyte fuel cell at a cell temperature of 70° C. anda current density of 0.2 A/cm² is evaluated, and the cell voltagebecomes 750 mV and the rate of voltage reduction becomes about 2 μV/hafter 1,000 hours from the beginning of the operation. Here, the gas tobe supplied to each of the anode and cathode is supplied to the cell asa gas moisturized to have a dew point of 70° C.

EXAMPLE 3

The 10% ethanol solution of the electrolyte polymer obtained in Example2, was cast on a substrate and then dried to obtain a cast membranehaving a thickness of 30 μm. The obtained membrane was subjected to heattreatment at 120° C. for 0.5 hour to obtain a membrane for fuel cells. Amembrane-electrode assembly was prepared in the same manner as inExample 2 except that this membrane was used, and its power generationproperties were measured. As a result, the cell voltage was 730 mV andthe rate of voltage reduction was about 2 μV/h, after 1,000 hours fromthe beginning of the operation.

EXAMPLE 4 (COMPARATIVE EXAMPLE)

Using a copolymer A not subjected to heat treatment or fluorinationtreatment, a test of immersing it in a fenton reagent was carried out inthe same manner as in Example 1. After such a test, the fluorine ionconcentration in the solution was measured by an ion meter, whereuponthe amount of eluted fluorine ions was calculated and found to be 0.063%of the total amount of fluorine in the polymer immersed.

EXAMPLE 5 (COMPARATIVE EXAMPLE)

Using a copolymer A which was subjected only to heat treatment inExample 1 without being subjected to fluorination treatment, a test ofimmersing it in a fenton reagent was carried out in the same manner asin Example 1. After such a test, the fluorine ion concentration in thesolution was measured by an ion meter, whereupon the amount of elutedfluorine ions was calculated and found to be 0.050% of the total amountof fluorine in the polymer immersed.

EXAMPLE 6 (COMPARATIVE EXAMPLE)

A membrane-electrode assembly was prepared and its power generationproperties were measured in the same manner as in Example 2 except thatthe polymer obtained in Example 5 was used. As a result, the cellvoltage was 695 mV and the rate of voltage reduction was about 80 μV/h,after 1,000 hours from the beginning of the operation.

EXAMPLE 7 (COMPARATIVE EXAMPLE)

A copolymer A is subjected to fluorination treatment, and a test ofimmersing it in a fenton reagent is carried out in the same manner as inExample 1, except that heat treatment is carried out under normalpressure. Measurement is carried out in the same manner as in Example 5,whereupon the amount of eluted fluorine ions in the solution after sucha test is calculated and found to be 0.005% of the total amount offluorine in the polymer immersed.

EXAMPLE 8 (COMPARATIVE EXAMPLE)

A membrane-electrode assembly is prepared and its power generationproperties are measured in the same manner as in Example 2 except thatthe polymer obtainable in Example 7 is used. The cell voltage becomes725 mV and the rate of voltage reduction becomes about 20 μV/h, after1,000 hours from the beginning of the operation.

INDUSTRIAL APPLICABILITY

The electrolyte polymer of the present invention has very few unstableterminal groups. Therefore, a polymer electrolyte fuel cell providedwith a membrane-electrode assembly having an electrolyte membranecomprising the electrolyte polymer of the present invention or amembrane-electrode assembly having a catalyst layer containing theelectrolyte polymer of the present invention, is excellent in thedurability, as the decomposition of the polymer due to fuel celloperation can be suppressed. Such a membrane-electrode assembly cansuitably be used not only for hydrogen-oxygen fuel cells, but also fordirect methanol type fuel cells or the like.

Further, according to the production process of the present invention, aperfluorinated polymer having sulfonic groups can be subjected tofluorination treatment efficiently and sufficiently, whereby the aboveelectrolyte polymer having very few unstable terminal groups can beefficiently obtained.

The entire disclosure of Japanese Patent Application No. 2003-133991filed on May 13, 2003 including specification, claims and summary isincorporated herein by reference in its entirety.

1. A process for producing an electrolyte polymer for polymerelectrolyte fuel cells made of a perfluorinated polymer having sulfonicgroups, characterized in that a perfluorinated polymer having precursorgroups for sulfonic groups, is subjected to heat treatment for at least0.1 hour at a temperature of from 200 to 300° C. under a reducedpressure of at most 0.02 MPa and then contacted with fluorine gas at atemperature of from 150 to 200° C. and further subjected to hydrolysisand treatment for conversion to an acid form, to convert the precursorgroups to sulfonic groups.
 2. The process for producing an electrolytepolymer for polymer electrolyte fuel cells according to claim 1, whereinthe electrolyte polymer is such that in a test of immersing 0.1 g of thepolymer in 50 g of a fenton reagent solution containing 3% of an aqueoushydrogen peroxide solution and 200 ppm of bivalent iron ions at 40° C.for 16 hours, the amount of eluted fluorine ions detected in thesolution is not more than 0.002% of the total amount of fluorine in thepolymer immersed.
 3. The process for producing an electrolyte polymerfor polymer electrolyte fuel cells according to claim 1, wherein theelectrolyte polymer is a perfluorocarbon polymer wherein the basicskeleton comprises repeating units based on CF₂═CF₂ and repeating unitsbased on CF₂═CF(OCF₂CFX)_(m)—O_(p)—(CF₂)_(n)SO₃H(wherein X is a fluorineatom or a trifluoromethyl group, m is an integer of from 0 to 3, n is aninteger of from 0 to 12, and p is 0 or 1, provided that when n=0, p=0).4. The process for producing an electrolyte polymer for polymerelectrolyte fuel cells according to claim 2, wherein the electrolytepolymer is a perfluorocarbon polymer wherein the basic skeletoncomprises repeating units based on CF₂═CF₂ and repeating units based onCF₂═CF(OCF₂CFX) _(m)—O_(p)—(CF₂)_(n)SO₃H(wherein X is a fluorine atom ora trifluoromethyl group, m is an integer of from 0 to 3, n is an integerof from 0 to 12, and p is 0 or 1, provided that when n=0, p=0).